Influence of the Electrolyte for CO2 electroreduction on Cu Catalysts

Abstract

Global warming and climate change due to emission of greenhouse gases such as CO2 have been recently a major threat to all living beings in the world. In this regard, CO2 electroreduction (CO2ER) has attracted considerable attention as it can mitigate the CO2 level in the atmosphere and produce value-added chemicals. Aqueous electrolytes are excellent choices for CO2ER due to non-toxicity, abundance, and low cost. However, the presence of the side hydrogen evolution reaction (HER) and low CO2 solubility in aqueous electrolytes have caused CO2ER to be inefficient. Electrolyte design has been reported as one of the methods to enhance the selectivity and activity. Electrolytes have the ability to enhance the selectivity and activity by altering the CO2 solubility, local CO2 concentration, pH, and conductivity. Additives can be a promising method to overcome the challenges of aqueous solutions. They can stabilize the intermediates on the surface and enhance CO2ER. In this study, the impact of the electrolyte composition on the CO2ER has been investigated by using electrochemical and spectroscopical methods.In order to study the impact of anions and cations in aqueous electrolytes, different salts (10 mM) with sodium (Na+) or 1-butyl-3-methylimidazolium [BMIM]+ as cation and bis(trifluoromethylsulfonyl)imide [NTF2]- or dicyanamide [DCA]- as anion were added to 0.1 M KHCO3. Results showed that a small amount of salts in the electrolyte can have a significant enhance on the selectivity and activity in CO2ER. NTF2-based electrolytes showed a high faradaic efficiency (FE%) for formate at -0.92 V vs. RHE likely due to high hydrophobicity and CO2 absorption capacity of the NTF2-salts. [BMIM][NTF2] showed the maximum FE (38.7%) for formate. This can be due to the interactions of [BMIM]+ with CO2 molecules and stabilizing the intermediates on the surface. On the other hand, [DCA]-based salts showed a high FE for hydrogen and a very low FE for hydrocarbons even at high overpotentials. We attributed this observation to the strongly adsorption of [DCA]- on the surface. X-ray photoelectron spectroscopy (XPS) also confirmed the strong adsorption of the [DCA]- anions on the surface. Strongly adsorbed DCA ions on the surface can promote hydrogen evolution reaction, destabilize the intermediates and suppress CO2ER. In-situ electrochemical quartz crystal microbalance (EQCM) also showed a higher mass loss for DCA-salts probably due to the displacement of water molecules by ions. Electrochemical impedance spectroscopy (EIS) showed that both cations and anions impact the solution resistance and charge transfer resistance. Comparing to [BMIM]+ containing electrolytes, Na-based salts showed a lower solution resistance due to the small size of Na+ ions and their high conductivity. However, the charge transfer resistance is more impacted by the anion nature. DCA salts showed a lower charge transfer resistance compared to NTF2 salts probably due to the enhanced HER which is kinetically faster than CO2ER. This study showed that how a small amount of additives can alter the selectivity and activity in CO2ER in aqueous electrolytes.